Method and apparatus for sensing gases



Dec. 10, 1968 J. w. M GOWAN, JR 3,

METHOD AND APPARATUS FOR SENSING GASES Original Filed June 5, 1964 5Sheets-Sheet 1 INVENTOR. James W. Mc Gowan, Jr. 8

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AT TORNEY Dec. 10, 1968 J. w. MCGOWAN, JR 3,416,070

METHOD AND APPARATUS FOR SENSING GASES Criginal File i June 5, 1964 3sheets-sheet 2 FIG.2

55 J: +2av vDef;- 1968 J. w. MCGOWAN, JD 3,

METHOD AND APPARATUS FOR SENSING GASES Original Filed June 5, 1964 3sheets'sheet 5 I FIG. 4

TEST GAS SOURCE a 2 GAS DETECTOR CHAMBER UNDER 88 1 TEST 86 so M FIGS 92GAS fHAMBER DETECTOR UNDER TEsT a 1 TEST GAS SOURCE g;

United States Patent 3,416,070 METHOD AND APPARATUS FOR SENSING GASESJames William McGowan, Jr., 810 Crest Road, Del Mar, Calif. 92014Continuation of application Ser. No. 372,984, June 5, 1964. Thisapplication Mar. 8, 1968, Ser. No. 711,811

' 30 Claims. (Cl. 324-33) ABSTRACT OF THE DISCLOSURE Apparatus fordetecting leaks in a vacuum system; the operation thereof being based onthe fact that some gases, e.-g. helium, rapidly pass through small leaksin a vacuum system and that these gases can be effectively exicted byelectron impact to a specific metastable state; the thus excited atom,which is unaffected by electric fields, can then drift to a collectorfor detection. However, ions and electrons which exist in the apparatusin large numbers, are effectively eliminated by electric fields. Suchapparatus with collectors of different materials, permits identificationof certain gases.

This application is a continuation of application Ser. No. 372,984,filed June 5, 1964, now abandoned.

Although there may be a number of instances where it may be necessary todetect the presence of a gas, such sensing procedures are particularlyimportant when leaks, particularly those of minute size are to belocated. Mass spectrometers have been found to be particularly usefulfor the purpose and can make precise and sensitive measurements in leakdetector operations. However, such apparatus is bulky and relativelyexpensive.

Accordingly, an object of this invention is to provide an improvedmethod for sensing the presence of a selected gas in a given region,which utilizes equipment of compact size, such equipment beingrelatively economical in cost.

Another object of this invention is to provide an improved method ofdetecting leaks in a system.

It is also an object of this invention to provide an improved method andapparatus for generating an electric current in response to the presenceof a selected gas in a given region.

Otherobjects of this invention will in part be obvious and in parthereinafter pointed out.

The instant invention contemplates the excitation of particles of a gasto metastable states of given energy and the interaction of suchmetastable excited gas particles with a collector which is connected toa source of electrons. The collector has a work function which has agiven relationship with respect to the metastable excited gas particles.In some, but not all cases, the work function is less than the availablemetastable energy of the gas particles. In such case, when a metastablyexcited gas particle interacts with the collector, electrons are removedtherefrom. The electrons which are liberated have a kinetic energywhich, at maximum, is the difference between the energy contained in themetastable particle and the work function. As each electron is removed,an electron is drawn from the source of electrons to replace the same.Accordingly, an electric current is established, which current flowsfrom the source of electrons to the collector.

The gas particles may be in either atomic or molecular form. Acollector, as herein mentioned is a surface material which can liberateelectrons and which can replace the liberated electrons by otherelectrons from a source of electrons to which it is connected. Thecurrent flow from the source of electrons to the collector is relateddirectly to the number of metastables striking it.

3,416,070 Patented Dec. 10, 1968 With the broad method of the inventionachieved, the same may be applied to detect the presence of selectedgases whose particles may be metastably excited, and further, may thusbe utilized to detect leaks.

FIG. 1 is a front elevational view, with parts broken away, of anapparatus for generating an electric current in accordance with oneembdiment of the invention;

FIG. 2 is a view showing a modified form of the device shown in FIG. 1;

FIG. 3 shows still another embodiment of the invention;

FIG. 4 is a schematic diagram showing one method of detecting leaks inan evacuated chamber in accordance with the invention;

FIG. 5 is a schematic diagram showing another method of detecting leaksin a chamber in accordance with the invention; and

FIG. 6 is a front elevational view, with parts broken away, andincluding schematic representations of apparatus for detecting aparticular gas in a mixture of gases.

FIG. 1 shows one embodiment of the invention which is useful forgenerating an electric current which may be used for detecting thepresence of a certain gas or gases. The gas detecting device comprisesan envelope 10 of an insulative material such as glass, which envelopehouses a plurality of electrical elements. Thus, the envelope insubstantially cylindrical form. has mounted therein a cathode 12 whichthrough resistive heating boils off electrons. Cathode 12 is fixed alongthe major axis of envelope 10'. Concentrically disposed about cathode 12is a first electrode 14 which acts as an electron accelerating means.

Electrode 14 is shown as a helical wire, although the same may take theform of a gridlike cylinder of wire mesh which is mechanically permeableto gas particles, electrons and ions. Concentrically disposed aboutelectrode 14 is an electrode 16, also in helix form, which acts as anelectron repeller element, electrode 16 being similar structurally toelectrode 14. Electrode 16 is effective to optimize the operation of thedevice.

An electrode 18, also in helix form, is concentrically disposed aboutelectrode 16, and functions as an ion repeller element; being similar asto structure to electrodes 14 and 16. A cylindrical collector electrode20 is concentrically disposed about electrode 18. Collector 20 is formedof a material adapted to donate electrons when interacted withmetastably energized gas particles, that is, the work function of thesurface is less than the energy in the metastable level.

A sealed feed through 22 is connected to one end of a resistor 24, theother end of which is connected to ground to provide a source ofelectrons, the feed through being also connected to collector 20.Cathode 12 is connected via sealed feed through 26 and 27 to a +5 voltsource of potential. In this manner a current is forced through cathode12 which beats it and starts boiling olf electrons.

Sealed feed through 30 connects electrode 14 to a 28 volt source ofpositive potential. Electrode 16 is grounded via seal through 32 whilesealed feed through 34 connects electrode 18 to a 30 volt source ofpositive potential. A gas inlet 36 and a gas outlet 38 extend fromenvelope 10. In some cases, the gas outlet 38 may not be required.

The device operates as follows: A gas having particles which can bemetastably energized is introduced into envelope by way of inlet 36 andpasses out by way of outlet 38. At this time, electrons boiled off fromcathode 12 are accelerated radially out-ward by electrode 14 and in thecourse of their flight they collide with the particles of the gas. Thecollisions will give rise to metastable particles of the gas which areelectrically neutral in addition to charged particles.

It is now necessary to make certain that only gas particles inmetastable states reach the collector 20, for reasons hereinafterappearing. Accordingly, cathode 12 is biased to a higher potential thanthat of collector 20, so that electrons will flow back to cathode 12.Electrode 18 which is at a potential higher than that of cathode 12,also repels positive ions back toward cathode 12 and electrodes 14 and16 and recombinations take place.

To enhance this charged particle filtering action, electrode 16 ismaintained at a potential less than that of electrodes 14 and 18. In theabsence of electrode 16, electrons would oscillate back and forth aboutelectrode 18. Any ionization collisions occurring in the region betweencollector 2t) and electrode 18 would generate positive ions which wouldbe picked up by collector 20. Accordingly, electrode 16 is effective toset up a strong repelling field for the electrons to insure a minimum ofelectron oscillations about electrode 18 and a maximum of suchoscillations about electrode 14 so that the collision region is on theside of the repeller electrode 18 remote from collector 20.

Since the metastable gas particles are electrically neutral, theirpassage is essentially unopposed except by the grids and they can movetoward the surface of collector 20. If the metastable energy of the gasparticle is at least equal to the work funcion of the collector 20, thenwhen the metastable particle contacts the surface of collector 20, itwill cause an electron to be ejected from the surface of collector 20and will be drawn toward the repeller electrode 18. Thus, electrode 18also serves to prevent an electron space charge from building whichwould limit the electron flow from the surface of collector 20.

As each electron is liberated. collector 20 draws an electron fromground via resistor 24, to replace the liberated electron. The flow ofelectrons from ground through resistor 24 provides a generatedelectrical current. The magnitude of such current is measured by meterM. It will now be seen that it is undesirable for electrons from thesource of electrons or ions formed by electron collision in the gas toimpinge on the surface of collector 20 because any charged particlesreaching the collector 20 would generate noise current.

The device shown in FIG. 2 illustrates another embodiment of theinvention and which is similar to that shown in FIG. 1, except ashereinafter pointed out. The elements 12, 14, 16, 18 and 20 of thedevice of FIG. 1, find their counterparts as cathode 12', electrodes14A, 14B; 16A, 16B; 18A, 18B; and collectors 20A, 20B. Thus, whereas theelectrodes 14, 16, 18 and collector 20 are in cylindrical form, theircounterpart electrodes are essentially paired planar screenssymmetrically related to cathode 12. The electrodes 14A, 14B; 16A, 16Band cathode 12' are located between casing members 11 which are spacedby suitable rods R and which are open at opposite sides defined byelectrodes 18A, 18B. Casing 11 provides shield means for the platecollectors 20A, 20B disposed opposite electrodes 18A, 18B, againstelectrons and ions. The region beyond electrodes 18A, 18B is open sothat gas may flow into the energizing region of the device. Theconnections to various potential sources and to ground, for the severalelectrodes, the cathode and collectors, is similar to that of FIG. 1.According to another feature of the invention, further electrodes 19A,19B are disposed between respectively collector 20A and electrode 18A,and collector 20B and electrode 18B. Electrodes 19A, 19B are connectedto a l volt potential and provide a metastable energy filter.

It will be recalled that the energy of the liberated electrons iseffectively the difference between the energy of the metastable particleand the work function of the collector. Therefore, if there are twodifferent kinds of metastable particles each with a different metastableenergy and both liberate electrons from the collectors 20A, 208, theliberated electrons will fall into two diffcrent groups with each grouphaving a different energy.

If the potential of electrodes 19A, 19B is chosen to provide a potentialbarrier to the low-energy liberated electrons but not the high-energyliberated electrons, lowenergy liberated electrons will be driven backto the collectors a and 20b to subtract from the generated current.However, the high-energy liberated electrons will pass over thepotential barrier. Accordingly, the high energy electrons are the onlyelectrons contributing to the measured current. Therefore, the measuredcurrent is an indication of the number of high energy metastableparticles.

FIG. 3 shows another embodiment of the invention wherein the severalelements are of cylindrical form. The cylindrical gridlike electrodes54, 5'6, 58 and 59 are disposed between circular insulating end walls51a, 51b. Rodlike supports, not shown, maintain the end walls 51a, 51bat a given spacing. A heated cathode 52 is located below a centralopening in an apertured shield 55 maintained at at +28 volt potentialand disposed in end wall 51b to provide a source of energizingelectrons. Cathode 52 is biased to a +5 volt potential.

A plate 53 is located opposite a central opening in end wall 5111, saidplate 53 being in axial alignment with cathode 52 and at a voltpotential. A solenoid 57 is wound about the assembly so as to provide anaxial magnetic field when said solenoid is energized. Cathode 52, shield55, plate 53 and solenoid 57 suitably energized, cooperate to form anelectron gun and target system wherein electrons from cathode 52 travelin helical paths about the central axis of the device to plate 53.Electrode 55 electrically and mechanically connected to grid 54, is at a+28 volt potential to accelerate the electrons boiled off cathode 52 tocause their collision with gas particles. Electrode 56 which isconnected to ground, acts as an electron repeller while electrode 58acts-to repel positive ions and to attract liberated electrons.Electrode 58 is connected to a +28 volt source. Electrode 59 connectedto a 1() volt source acts as an energy filter in the same manner aselectrodes 19a and 19b operate for the device of FIG. 2. Collector 60which is connected via resistor 64 to ground which provides a source ofelectrons, operates in the manner described for collectors 20 and 20.

With the device shown in FIG. 3, the collision path is of increasedlength, there is a reduced likelihood of electrons reaching the surfaceof collector 60 and the energizing electrons are within a narrower rangeof energies.

FIG. 4 shows one method for detecting a leak in a chamber. Thus chamberunder test, is connected via a conduit 86 to a gas detector 88. Detector88 may take the form of any one of the devices shown in FIGS. 1-3. Atest gas which can be excited to metastable states from a source 82 isdispensed by a flexible conduit 84 about the outer surface portions ofchamber 80 and particularly in the region of seals or seams therein. Ifthere is any leak in chamber 80, the test gas will enter the same andpass outwardly thereof by way of conduit 86 into leak detector 88. Thepresence of such gas will be detected, in the manner previouslydescribed, causing a current to flow and to register on meter M whichwill be deflected from its quiescent position.

Alternatively, leaks may be detected in the manner illusrtated by FIG.5. Here, metastably excitable test gas from source 82 is passed intochamber 80 undergoing test, by way of conduit 90. A gas collector orsniffer 92 is passed over the exterior surface of chamber 80,particularly in the region of seals or seams, and any leaking gas willbe collected by sniffer 92 and passed to detector 88, giving rise to anelectric current which registers on meter M.

The devices disclosed herein are highly effective when the test gas ishelium. Neon and argon may also be used. Helium is preferred since itoccurs in the atmosphere in extremely minute quantities, and in lesserproportions than argon for example. The energies of helium metastablesis of the order of 20 electron volts so that the electrons liberatednormally have high kinetic energy requiring little field to draw themfrom the collector. The metastable levels of other atmospheric gases areconsiderably lower so that electrons liberated by these metastables haveproportionally less kinetic energy than do those from heliummetastables. A low draw out field from elements 58 or 18 thusdiscriminates against other metastables. The discrimination is enhancedwhen electrodes such as electrodes 19a and 19b and electrode 59 areemployed.

To further discriminate against inaccuracies introduced by commonatmospheric gases, it is desirable to make the collectors 20, 20', 60 ofsurface materials having a relatively high work function, such asplatinum, nickel, gold or silver. Helium gas and a platinum surfacecollector provides a good combination. However, other combinations ofgas and metal may be used, providing the metastable energy of the gasparticle is greater than the work function of the collector surface. Itshould be noted that energy filter electrodes such as 19a and 19b and 59broaden the range of possible combinations that provide the requireddiscrimination.

FIGURE 6 shows a metastable spectrometer or apparatus for detecting aparticular gas within a mixture of gases. The operation of suchapparatus depends on the fact that the particular gas has a metastablestate of given energy such as E. The metastable particles containingenergy E can cause the removal of electrons from a collector surfacehaving a work function E which is less than E but cannot cause theremoval of electrons from a collector surface having a Work function Ewhich is greater than B. Accordingly, if a mixture of gases ismetastably excited and the metastable particles allowed to react withcollector surfaces having the above described different work functions,the presence of the particular gas can be detected by measuring for adifference in the current flow to both collectors. Provided no physicalbiases are present in the system, except the difference in workfunctions and the associated difference in secondary electron emission,the current flow to the collector with work function E will be relatedto the number of gas particles n in metastably excited. states withenergy EZE Similarly, the current flow to a second collector having workfunction E will be related to the number of gas particles n inmetastably excited states with energies EzE Therefore, the difference incurrent flow from ground to these plates is related to the difference712-111.

The apparatus of FIG. 6 is adapted to measure the effective numberdifference. This apparatus is similar to the apparatus of FIG. 2 exceptas indicated. Accordingly, the common structure will not be redescribedand only the differences are noted. Collector 21a has a surface with awork function E greater than the work function E of collector 21b. Itshould be noted that the modification of apparatus of FIG. 2 is by wayof example. Similar modifications may be applied to the device of FIGS.1 and 3 wherein the cylindrical collectors can be divided into two equalparts that are electrically insulated from each other, each of the partshaving a surface with the different work functions E and E.

In any event, collector 21a is connected to ground via resistor 23 andcollector 21b is connected to ground via resistor 25. The voltage acrossresistor 23 is fed to one input of difference amplifier 27, ofconventional design, and the voltage across resistor 25 is fed to theother input of the difference amplifier 27.

When the apparatus is in an atmosphere of gases under test, the gasesare energized by electrons as described above. The metastable particlesof the gas now contact the collectors 21a and 21b causing the abovedescribed current flow. Since the efficiencies for production ofsecondary electrons from two different surfaces with work functions Eand E less than the metastable energy E is usually different, theresistors 23 nad 25 must be chosen such that the potentials developedacross them are nearly equal. However, when E E E there is a potentialdeveloped across resistor 23 and not across resistor 25 which gives riseto an imbalance as measured on the ammeter 29 associated with thedifference amplifier 27. Such an imbalance consequently indicates thepresence of metastables with an energy between E and E It is clear thatthe metastable spectrometer need not be limited to two differentcollector materials of different work functions. In practice, thespectrometer might have a multiplicity of different surfaces.

As various changes might be made in the embodiment of the inventionherein shown with departing from the spirit thereof, it is understoodthat all matter herein described or illustrated is not limiting exceptas set forth in the appended claims.

I claim:

1. A method of detecting the presence of a given gas which can beexcited to metastable states in a given region comprising bombardingsaid region with particles of high energy content to excite the gasparticles in said region to at least metastable states of givenenergies, engaging said excited gas particles with the surface of acollector connected to a source of electrons and having a work functionless than the magnitudes of said given energies whereby electrons areremoved from the surface of said collector by said metastably excitedgas particles and replaced by an electric current flow from said sourceof electrons to said collector and registering said current flow toindicate the presence of said given gas.

2. A method for detecting the presence of a particular gas in a mixtureof metastably excitable gases in a given region comprising the steps ofbombarding said region with particles of high energy content to excitethe gas particles in the region to metastable states wherein themetastable energy of each of the gases is different, allowing themetastably excited gas particles to contact the surfaces of first andsecond collectors connected to a source of electrons the surface of saidfirst collector having a work function which is less than the metastableenergy of the particles of the particular gas and the surface of saidsecond collector having a work function which is greater than themetastable energy of the particles of the particular gas wherebyelectrons are removed from the surfaces of said collectors and replacedby an electric current flow from said sources of electrons to saidcollectors, and measuring the difference in the current flow to saidcollectors to indicate the presence of the particular gas.

3. A method for detecting a given gas comprising the steps of passingthe gas through a zone traversed by particles having high kinetic energyto excite the gas into metastably excited states, filtering out theparticles, engaging a source of electrons by said metastably excitedgas, and measuring the flow of electrons produced by said engagement.

4. A gas detector comprising an electron source, an electron anodedisplaced from said electron source, said source and anode defining anaxis, means for accelerating electrons along said axis from electronsource to said electron anode, a first electrode disposed about saidaxis, a second electrode disposed about said first electrode, a thirdelectrode disposed about said second electrode, a collector disposedabout said third electrode, means for connecting said collector to asource of electrons, means for maintaining said first electrode at apotential greater than that of said electron source, means formaintaining said second electrode at a potential less than that of saidfirst electrode, means for maintaining said third electrode at apotential gerater than that of said second electrode and of saidcollector, an envelope of non-porous material encompassing said electronsource, anodes and said eletrodes, means for establishing a magneticfield colinear with said axis and means for introducing a gas into saidenvelope.

5. A device as defined in claim 4 wherein the nonporous material isglass.

6. The gas detector of claim 4 further comprising a fourth electrodedisposed about said third electrode and means for maintaining saidfourth electrode at a potential less than that of said collector.

7. A gas detector comprising an envelope, a cathode within saidenvelope, a first electrode in displaced relation to said cathode, asecond electrode in displaced relation to said first electrode, andremote from said cathode, a third electrode in displaced relation tosaid second electrode and remote from said first electrode, first andsecond collectors displaced from said third electrode and remote fromsaid second electrode, the surfaces of said collectors having differentwork functions, means connecting said collectors to a source ofelectrons, means for maintaining said cathode at a potential greaterthan that of said collectors, means for maintaining said first electrodeat a potential greater than that of said cathode, means for maintainingsaid second electrode at a potential less than that of said firstelectrode, means for maintaining said third electrode at a potentialgreater than that of said second electrode and said collectors and meansfor introducing a gas into said envelope.

8. A gas detector comprising an envelope, means in said envelope forproducing a zone of particles having a high kinetic energy, means forpassing a gas into said envelope and through said zone to excite the gasinto at least one metastably excited state, an electron source spacedfrom said means for producing the high energy particles, and filteringmeans between said source and the means for producing the high energyparticles to filter the high energy particles from the source.

9. A device as defined in claim 8 further including means for exhaustingthe gas from the envelope.

10. A device as defined in claim 8 further including means connected tothe electron source for measuring the fiow of electrons therein.

11. A device as defined in claim 8 wherein the means for producing thezone of high kinetic energy particles comprises a cathode, an anodespaced from said cathode, and means secured to said anode and cathodefor connection to a source of electrical energy.

12. A device as defined in claim 11 wherein the cathode comprises aconductor and the anode comprises a shield permeable to gas particlessurrounding the cathode.

13. A device as defined in claim 10 wherein the shield is a helicalwire.

14. A device as defined in claim 11 wherein the anode comprises a plateand the cathode comprises a heater axially spaced therefrom.

15. A device as defined in claim 12 wherein the filtering meanscomprises a first electrode surrounding the shield and a secondelectrode surrounding the first elec trode.

16. A device as defined in claim 15 wherein the electron sourcecomprises a collector surrounding the second electrode.

17. A device as defined in claim 16 wherein said collector has a surfaceof platinum.

18. A device as defined in claim 16 wherein said collector has a surfaceof nickel.

19. A device as defined in claim 16 wherein said collector has a surfaceof gold.

20. A device as defined in claim 16 wherein said collector has a surfaceof silver.

21. A device as defined in claim 16 further including means applying apositive potential to the anode and second electrode and means applyinga lower potential to the collector and first electrode.

22. A device as defined in claim 11 wherein the anode comprises a pairof planar shields permeable to gas and the cathode comprises a conductorbetween the shields.

23. A device as defined in claim 22 wherein the filtering meanscomprises a first pair of electrodes adjacent the pair of shields and asecond pair of electrodes adja- 0 cent the first pair of electrodes.

24. A device as defined in claim 23 wherein the electron sourcecomprises a pair of plates spaced from the second pair of electrodes.

25. A device as defined in claim 24 further including a third pair ofelectrodes between the second pair of electrodes and the plates.

26. A device as defined in claim 25 wherein the plates have a surface ofa material selected from the group consisting of platinum, nickel, goldand silver.

27. A device as defined in claim 25 further including means applying apositive potential to the anode and second electrodes, means applying anegative potential to the third electrodes, and means applying a commonpotential between said aforementioned positive and negative potentials,to the pair of plates and first pair of electrodes.

28. A device as defined in claim 27 further including means connected tothe plates for measuring the flow of electrons therein.

29. A device as defined in claim 24 wherein one of the plates has a workfunction greater than the other of the plates and further including adifference amplifier, means connecting each plate to an input of saidamplifier, and means for measuring the current output of the amplifier.

30. A device as defined in claim 14 wherein the filtering meanscomprises three concentric electrodes and the electron source comprisesa collector surrounding the outermost electrode, a solenoid surroundingthe collector, and means applying a positive potential to the twooutermost electrodes and means applying a lower potential to thecollector and innermost electrode.

References Cited UNITED STATES PATENTS 2,516,704 7/1950 Kohl 32433 X2,963,601 12/1960 Varnerin et al. 324-33 X 3,001,128 9/1961 Nottingham324-33 3,126,512 3/1964 Zito 324-33 3,267,326 8/1966 Hayward et a1.3,271,731 9/1966 Adler et al 3l3-7 X RUDOLPH V. ROLINEC, PrimaryExaminer.

C. F. ROBERTS, Assistant Examiner.

